Your indentation is confusing. Please indent the continuation line more than its parent. Also, it's better to avoid explicit continuation lines (with `\`) and use parentheses instead. Here, you could put parentheses around the whole equality, as in:

```python
assert (simplify(Product(x, (x, n, m)) * Product(x**2, (x, n, m))) ==
    Product(x**3, (x, n, m)))
```